Recently, as information and communication technology rapidly develops, a ubiquitous society based on information and communication technology is being formed.
To allow information communication devices to be connected anytime and anywhere, sensors equipped with a computer chip having a communication function should be installed in all facilities. Therefore, supply of power to these devices or sensors is a new challenge. In addition, as the kinds of portable devices such as Bluetooth handsets and music players like iPods, as well as mobile phones, rapidly increase in number, charging batteries thereof has required time and effort. As a way to address this issue, wireless power transmission technology has recently drawn attention.
Wireless power transmission (or wireless energy transfer) is a technology for wirelessly transmitting electric energy from a transmitter to a receiver based on the induction principle of a magnetic field. Back in the 1800s, electric motors or transformers based on electromagnetic induction began to be used. Thereafter, a method of transmitting electric energy by radiating a high frequency wave or an electromagnetic wave, such as a microwave or laser, was tried. Electric toothbrushes and some common wireless shavers are charged through electromagnetic induction.
Wireless energy transmission techniques introduced up to now may be broadly divided into magnetic induction, electromagnetic resonance, and RF transmission employing a short wavelength radio frequency.
In the magnetic induction scheme, when two coils are arranged adjacent each other and current is applied to one of the coils, a magnetic flux generated at this time generates electromotive force in the other coil. This technology is being rapidly commercialized mainly for small devices such as mobile phones. In the electromagnetic induction scheme, power of up to several hundred kilowatts (kW) may be transmitted with high efficiency, but the maximum transmission distance is less than or equal to 1 cm. As a result, devices are generally required to be placed adjacent to a charger or a pad, which is disadvantageous.
The magnetic resonance scheme uses an electric field or a magnetic field instead of employing an electromagnetic wave or current. The magnetic resonance scheme is advantageous in that the scheme is safe for other electronic devices or the human body since it is hardly influenced by the electromagnetic waves. However, the distance and space available for this scheme are limited, and the energy transfer efficiency of the scheme is rather low.
The short-wavelength wireless power transmission scheme (simply, RF transmission scheme) takes advantage of the fact that energy can be transmitted and received directly in the form of radio waves. This technique is an RF-based wireless power transmission scheme using a rectenna. A rectenna, which is a compound word of antenna and rectifier, refers to a device that converts RF power directly into direct current (DC) power. That is, the RF scheme is a technique of converting AC radio waves into DC waves. Recently, with improvement in efficiency, commercialization of RF technology has been actively researched.
The wireless power transmission technique is employable in various industries including IT, railroads, and home appliances as well as the mobile industry.
Recently, wireless power transmitters equipped with a plurality of coils have been introduced to increase the recognition rate of a wireless power receiver placed on a charging bed. However, the conventional wireless power transmitter equipped with a plurality of coils sequentially transmits detection signals, including, for example, a ping signal used for the electromagnetic induction scheme and a beacon signal used for the electromagnetic resonance scheme.
In particular, the conventional wireless power transmitter equipped with a plurality of conventional transmission coils is controlled to repeatedly transmit a detection signal sequentially through the respective transmission coils a predetermined number of times, for example, twice, to reduce recognition errors for the wireless power receiver and to determine transmission coils exhibiting good charging efficiency.
However, in the method of repeatedly transmitting the detection signal sequentially a predetermined number of times through each of the transmission coils, a transmission coil to be used for the wireless power receiver is identified only after a preset detection signal transmission procedure is completed. Therefore, it takes a lot of time to recognize the wireless power receiver. Moreover, the power of the capacitor of the charged wireless power receiver is discharged through the detection signal, and thus the wireless power receiver fails to transmit a predetermined response signal including a signal strength indicator to the wireless power transmitter. As a result, the receiver fails to be recognized.